### Bitcoin Optech Newsletter #84

This week’s newsletter seeks help testing a Bitcoin Core release candidate and summarizes some discussion about the BIP119 OP_CHECKTEMPLATEVERIFY proposal. Also included is our regular section about notable code and documentation changes.

## Action items

• Help test Bitcoin Core 0.19.1rc1: this upcoming maintenance release includes several bug fixes. Experienced users are encouraged to help test for any regressions or other unexpected behavior.

## News

• OP_CHECKTEMPLATEVERIFY (CTV) Workshop: video (morning, afternoon) and a transcript are available from a recent workshop about BIP119 CTV. If this proposed soft fork is adopted, users would be able to use a new CTV opcode to create covenants with less interaction than would be required using current consensus rules. Several possible applications of the opcode were discussed, with most attention being focused on vaults and compressed payment batching (sometimes called congestion control transactions, see Newsletter #48). A significant part of the workshop also consisted of critical feedback from the audience and replies to the criticism. A final discussion covered how and when to attempt to get BIP119 activated, including when a PR for it should be opened to the Bitcoin Core repository, what activation mechanism it should use (e.g. BIP9 versionbits), and what range of activation dates would be appropriate if it uses a miner-activated soft fork mechanism such as BIP9.

Subsequent to the workshop, CTV proposer Jeremy Rubin announced a mailing list to help coordinate future review and discussion of the BIP119 proposal.

## Notable code and documentation changes

Notable changes this week in Bitcoin Core, C-Lightning, Eclair, LND, libsecp256k1, Bitcoin Improvement Proposals (BIPs), and Lightning BOLTs.

• Bitcoin Core #17585 deprecates the label field returned by the getaddressinfo RPC as the labels (plural) field already exists and provides the same functionality. The label field is expected to be removed in 0.21; for compatibility, the old behavior can be re-enabled in the interim by launching bitcoind with -deprecatedrpc=label. This change is the final one in a series of changes to clean up the getaddressinfo RPC interface (including a PR covered in Newsletter #80).

• Bitcoin Core #18032 extends the results of the createmultisig and addmultisigaddress RPCs to include a descriptor field that contains an output script descriptor for the created multisig address. Providing a descriptor here makes it easier for the user (or a program that is calling this RPC) to get all the information they need to not only monitor for payments received to the created address but to also later create unsigned transactions which start the process of spending that money.

• C-Lightning #3475 allows a plugin hook to return { "result" : "continue" } to tell lightningd to process the action the same way it would without the hook being executed. This makes it easy for hooks to only execute in special cases.

• C-Lightning #3372 allows the user to specify an alternative program to use instead of one of the default sub-daemons (the C-Lightning system consists of multiple interacting daemons, referred to as sub-daemons of lightningd). For example (from the PR description):

# Use remote_hsmd instead of lightning_hsmd for signing:
lightningd --alt-subdaemon=lightning_hsmd:remote_hsmd ...


This option can be dangerous if the alternative sub-daemon isn’t fully compatible with the other daemons being used, but it also allows improved flexibility and may simplify some testing.

• C-Lightning #3465 implements anti fee sniping for withdrawal transactions, similar to LND’s implementation of the same thing (see Newsletter #18). Anti fee sniping uses the nLockTime field to prevent a transaction from being included in any block whose height is lower than that of the tip of the block chain when the transaction was produced. This limits the ability of a miner who is reorganizing (forking) the chain from being able to arbitrarily rearrange transactions to maximize their fee revenue.

• LND #3957 adds some code that can be used in later PRs that add Atomic Multipath Payments (AMP). AMP is another type of multipath payment similar to the “base” or “basic” type already supported by C-Lightning, Eclair, and LND. AMP is harder for routing nodes to distinguish from normal single-part payments and can guarantee that the receiver either claims all parts of the payment or none of it.

• BOLTs #684 updates BOLT7 to suggest that nodes send their own generated announcements even when the remote peer requests a filter that would suppress that announcement. This can help ensure that a node gets announced to the network via its direct peers without otherwise changing how filtering works.